When foods are added to a container ready to be filled it is required that the container is sterilized in such a way that the likewise sterilized product under sterile conditions can be added aseptically, the container then being sealed. In order to minimize the risk of the filled product being reinfected by microorganisms the sterilization of the container and the addition of the sterilized filling material in question should take place as closely connected as possible.
Containers ready to be filled in the form of a bowl or a can are presently sterilized by means of treatment with steam, steam/condensation or hydrogen peroxide. However, containers manufactured from a soaking paper or cardboard layer rapidly lose their mechanical strength properties, and thus a container easily becomes flabby and cumbersome when it is exposed to liquid or moisture during the sterilization procedure. When chemical sterilization with hydrogen peroxide is used, for example a 3% solution, the hydrogen peroxide is sprayed on all the internal surfaces of the container, and then is allowed to act for about 20 minutes. The hydrogen peroxide is converted to water and oxygen in course of time but problems with residual amounts can remain in the same way as when other types of chemical sterilization are used.
An electromagnetic radiation can also--in dependence of dose and energy--harm microorganisms and viruses in different ways. However, the practical use of this type of sterilization is limited by the ability of the radiation to penetrate and the availability of a suitable radiation source.
The sterilizing effect of electrons has been known for a long time. An exposure to electrons is a well known sterilization method and the mechanism behind the killing is thoroughly studied. However, the ability of the electrons to penetrate a package material is relatively low but depends of course on the radiation energy. Electron guns have been developed for food containers with the purpose of sterilizing relatively thin containers from the outside, and such a sterilization method has for several years been contemplated for an additional use within the packaging industry.
Up to now however, a problem with this type of sterilization has been the large volume of the equipment since electron guns having a large capacity will occupy a space of up to 1-5 m.sup.3. Furthermore, the equipment has been too expensive to be commercially acceptable, the costs amounting to 5 millions of SEK per unit. This has led to it being impossible to manufacture commercial sterilization equipments based on electron guns.
An additional problem with the existing electron guns is that they work at such a high voltage as about 150 kV or more. Since X-rays are generated as a secondary effect when electron guns are used a shield of lead must be used to protect the personal, which shield has to be properly enclosed. It would thus be of great advantage if an equipment containing lead could be avoided when foods are produced, such as for example in a dairy.
It is thus desirable that the present problems mentioned above could be avoided concerning the sterilization of containers which are ready to be filled. There is at the same time a need for a fast sterilization method in which the sterilization step reduces the cycle time of the sterilization process.